1. Technical Field
This invention relates generally to a method for removing sulfur from a lean NOx trap while simultaneously regenerating a Diesel particulate filter, and more particularly to such a method that controls the duration and frequency of engine operation in a rich combustion mode concurrently with control of the air/fuel ratio to provide the high temperature requirements of lean NOx trap desulfation and the oxidation of particles trapped in a Diesel particulate filter.
2. Background Art
Worldwide emissions regulations slated for introduction in the near future impose very stringent emissions regulations. The Tier 2 regulations in the United States require that Diesel vehicles have the same ultra-low emissions levels as spark ignited vehicles. Combustion mode changes, to address both in-cylinder (engine-out) and exhaust gas treatment device requirements have been proposed. For example, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a method by which the normal fuel lean operating mode of an engine is switched to a rich premixed charge compression ignition (PCCI) combustion mode.
U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for INTERNAL COMBUSTION ENGINE describes an alternative method for lowering the combustion temperature, i.e., low temperature combustion (LTC) to minimize smoke generation during rich, or near rich, combustion. LTC and PCCI combustion are alternative combustion modes which normal Diesel lean combustion can be transitioned to during engine operation.
Perhaps of most concern to the Diesel engine market are the proposed very tight future reductions in terms of oxides of nitrogen (NOx) and particulate matter (PM) emissions. One of the most promising technologies for NOx treatment is a NOx adsorber, also known as a “lean NOx trap.” Diesel particulate filters, also known as Diesel particulate traps, and lean NOx traps are the most likely, at least in the foreseeable future, means by which emissions will be reduced. Lean NOx traps and Diesel particulate filters need to be regenerated periodically to restore their efficiencies. The regeneration of lean NOx traps is usually done by providing reductants, such as CO and HC under oxygen-free conditions. A regenerated lean NOx trap not only adsorbs NOx emissions, but also adsorbs sulfur carried in the exhaust gas stream. Sulfur removal (desulfation) must be undertaken at a temperature above 600° C. under oxygen-free conditions, i.e., a stoichiometric or richer air/fuel ratio. Under typical Diesel lean combustion operation, such very high temperatures cannot normally be obtained except under very high load conditions. Diesel particulate filter regeneration is carried out by oxidizing soot and other particles “trapped” in the Diesel particulate filter at a high temperature and a lean air/fuel ratio.
Thus it can be seen that both desulfation of a lean NOx trap and regeneration of a Diesel particulate filter require very high temperatures. However, typical Diesel combustion cannot provide high exhaust gas temperatures because the engine operates with a lean to very lean combustion mixture. Heretofore, post-injection or in-exhaust injection has been used to obtain the required aftertreatment device regeneration temperatures. Post-injection can result in undesirable oil dilution due to wetting of the cylinder liner, and in-exhaust injection requires extraneous hardware. Morever, the desulfation process requires a substantially oxygen free atmosphere, whereas Diesel particulate filter regeneration is an oxidization process. Heretofore, these conflicting requirements have, of necessity, been carried out in separate operations.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a method by which desulfation of the a lean NOx trap (LNT) is carried out concurrently with oxidizing trapped particulate matter in a Diesel particulate filter (DPF).